A paramount concern in designing circuits for signal processing is the elimination or reduction of noise. Due to the broad diversity of desired source signals for processing and types of noise inherent thereto, a plethora of noise reduction systems have been developed. These prior art systems range from simple filters to program controlled signal enhancement computers. For the most part, these prior art noise reduction systems address the problem of reducing noise in a signal with a non-repeating waveform of significant complexity. An example would be an audio track from a tape source or LP. An audio signal source forms an essentially non-repeating waveform comprised of many different frequency components forming the desired signal and inherent noise. To reduce the noise content, circuits act to expand and compress segments of the audio signal, thereby selectively depressing those frequencies most poisoned by the noise. Unfortunately, this approach also depresses or eliminates portions of the source signal, thus reducing the quality of the output at the same time noise is removed.
There are important source signals that have a repeating complex waveform. For example, in the medical field, cardiologists are extremely interested in the electrical signals generated by the heart muscle during blood pumping operations. This signal is known as an electrocardiogram or "ECG" and has a repeating complex waveform associated with each beat of the heart. This complex waveform has undergone significant study by cardiologists. The waveform has been segmented into key portions for detailed study. It has been learned that various changes in the waveform are related to changes in the heart function. In fact, changes in certain segments of the waveform often act as a predictor to heart trouble, and therefore, are critical in diagnosis and prevention of heart disease.
The importance of ECG in the monitoring of the health of the heart cannot be overstated. Readings of the patient's ECG have now become a routine practice for the cardiologist, often taken right in his office. In this regard, the cardiologist will normally employ an oscilloscope or similar device for direct display of the ECG waveform, and ultimately a time graph printout. The cardiologist will review the ECG print-out looking for problems. The cardiologist may also compare the present ECG printout with a reading taken earlier. Differences in the two ECG signals often will indicate a health problem. For example, an elevated or depressed ST segment in the ECG reflects a certain type of heart valve disorder. The importance of the ST segment is more fully explained in U.S. Pat. No. 4,546,776 which is incorporated by reference herein in its entirety.
As noted in the above-referenced U.S. patent, the changes in the ECG waveform can be difficult to discern by the human eye. In addition, there are many potential sources of noise that further act to cloud the slight changes in the signal that are important in proper diagnosis. These noise sources include noise inherent in all electrical circuits, in addition to signal artifacts caused by patient movement, spurious electrical discharge unrelated to heart function, and sources in the doctor's office (e.g. fluorescent lights).
Noise induced artifacts in the ECG further inhibit a proper reading by the cardiologist causing erroneous diagnosis of life threatening conditions. It was with this understanding of the present problem the present invention was developed.